PHP上传后创建缩略图

I have implemented a file upload for pictures on my page, and tried to somehow generate thumbnails with the intention of clicking them to view via fancybox. The upload works but my function to create a thumbnail doesn't. (This is included in my upload.php, right after "move_uploaded_file":

<?php
$src = $subdir.$fileupload['name'];

function make_thumb($src) 
{

$source_image = imagecreatefromjpeg($src);    //For testing purposes only jpeg now
$width = imagesx($source_image);
$height = imagesy($source_image);
$desired_width = 220;

$desired_height = floor($height * ($desired_width / $width));

$virtual_image = imagecreatetruecolor($desired_width, $desired_height);

imagecopyresampled($virtual_image, $source_image, 0, 0, 0, 0, $desired_width, $desired_height, $width, $height);

header("Content-type: image/jpeg");
imagejpeg($virtual_image, realpath('./Thumbnails/filename.jpg'));    //Temporary filename, will be changed
}
?>

Just FYI, this is an assignment and since I am a php beginner, I did use google, but can't find the problem in my case. Maybe my understanding of php is lacking too much.

Use this img_resize function, it is good for the most popular image formats

   function img_resize($src, $dest, $width, $height, $rgb = 0xFFFFFF, $quality = 100)
   {
        if (!file_exists($src))
            return false;

        $size = getimagesize($src);

        if ($size === false)
            return false;

        $format = strtolower(substr($size['mime'], strpos($size['mime'], '/') + 1));
        $icfunc = "imagecreatefrom" . $format;
        if (!function_exists($icfunc))
            return false;

        $x_ratio = $width / $size[0];
        $y_ratio = $height / $size[1];

        $ratio = min($x_ratio, $y_ratio);
        $use_x_ratio = ($x_ratio == $ratio);

        $new_width = $use_x_ratio ? $width : floor($size[0] * $ratio);
        $new_height = !$use_x_ratio ? $height : floor($size[1] * $ratio);
        $new_left = $use_x_ratio ? 0 : floor(($width - $new_width) / 2);
        $new_top = !$use_x_ratio ? 0 : floor(($height - $new_height) / 2);

        $isrc = $icfunc($src);
        $idest = imagecreatetruecolor($width, $height);

        imagefill($idest, 0, 0, $rgb);

        if (($format == 'gif') or ($format == 'png')) {
            imagealphablending($idest, false);
            imagesavealpha($idest, true);
        }

        if ($format == 'gif') {
            $transparent = imagecolorallocatealpha($idest, 255, 255, 255, 127);
            imagefilledrectangle($idest, 0, 0, $width, $height, $transparent);
            imagecolortransparent($idest, $transparent);
        }

        imagecopyresampled($idest, $isrc, $new_left, $new_top, 0, 0, $new_width, $new_height, $size[0], $size[1]);

        getResultImage($idest, $dest, $size['mime']);

        imagedestroy($isrc);
        imagedestroy($idest);

        return true;
    }

    function getResultImage($dst_r, $dest_path, $type)
    {
        switch ($type) {
            case 'image/jpg':
            case 'image/jpeg':
            case 'image/pjpeg':
                return imagejpeg($dst_r, $dest_path, 90);
                break;
            case 'image/png';
                return imagepng($dst_r, $dest_path, 2);
                break;
            case 'image/gif';
                return imagegif($dst_r, $dest_path);
                break;
            default:
                return;
        }
    }